The present invention relates to electron beam irradiation apparatus and method, and more particularly to electron beam irradiation apparatus and method which are employed for irradiating combustion exhaust gas discharged from thermal power stations or the like with an electron beam to remove toxic components from the exhaust gas.
As economy develops, more and more energy is demanded. Amidst the continuous growth of energy demand, energy source is still dependent on fossil fuels such as coal and petroleum. However, the harmful products or pollutants generated by burning of fossil fuels are responsible for global pollution. It is considered that a global issue of the global warming and the acid rain caused by air pollution is attributed to components such as SOx and NOx which are contained in combustion exhaust gas discharged from thermal power stations or the like. As a method for removing toxic components such as SOx and NOx, there has been used a method of irradiating combustion exhaust gas with an electron beam for desulfurization and denitration (i.e. removing toxic components such as SOx and NOx).
In a flue gas treatment system for treating the combustion exhaust gas by employing an electrom beam, molecules such as oxygen (O2) and water vapor (H2O) in the combustion exhaust gas are irradiated with the electron beam emitted from an irradiation window comprising a thin film made of Ti or the like to form radicals such as OH, O, and HO2 having high oxidizing strength. These radicals oxidize toxic components such as SOx and NOx to produce sulfuric acid and nitric acid as intermediate products. These intermediate products react with ammonia gas (NH3) previously injected into the exhaust gas to produce ammonium sulfate and ammonium nitrate which are recovered as materials for fertilizer. Therefore, such a system for treating exhaust gas can remove toxic components such as SOx and NOx from the combustion exhaust gas and simultaneously recover ammonium sulfate and ammonium nitrate as useful by-products used for materials for fertilizer.
FIG. 3 shows an electron beam irradiation apparatus used for the above flue gas treatment system according to an example.
The electron beam irradiation apparatus 11 mainly comprises a thermoelectron generator 12 comprising a filament or the like, an accelerating tube 13 for accelerating electrons emitted from the thermoelectron generator 12, a focusing electromagnet 16 for controlling a diameter of the electron beam by applying the magnetic field to the high-energy electron beam formed in the accelerating tube 13, and scanning electromagnets 17, 18 for deflecting the electron beam in x and y directions by applying the magnetic field to the electron beam whose diameter has been controlled by the focusing electromagnet 16. The x direction is a horizontal direction shown in FIG. 3, and the y direction is a direction perpendicular to the x direction and also perpendicular to the sheet surface of FIG. 3. A surrounding comprising a container 19 and an irradiation window 20 is provided, and the interior of the surrounding is kept under high vacuum condition in the range of 1.33xc3x9710xe2x88x923 to 1.33xc3x9710xe2x88x924 Pa (10xe2x88x925 to 10xe2x88x926 Torr). The high-energy electron beam formed by the accelerating tube 13 is deflected and scanned by the scanning electromagnets 17, 18 which apply the magnetic field to the electron beam, and emitted through the irradiation window 20 into a certain range of an exhaust gas passage (not shown in FIG. 3) located at the outside.
Thermoelectrons generated by the thermoelectron generator 12 comprising a filament or the like are accelerated by high-voltage of about 800 kV, for example, in the accelerating tube 13 to cause a high-speed electron beam to be formed. Then, a beam diameter of the electron beam is controlled by the focusing electromagnet 16 to thus form a linear electron beam, having substantially the same diameter in a travelling direction in an example shown in FIG. 3, which is then directed toward the magnetic field formed by the scanning electromagnets 17, 18. The focusing electromagnet 16 comprises an electromagnet having a ring-shaped coil disposed around an axis of the electromagnet, and forms a magnetic field which is symmetric with respect to the axis of the electron beam. The beam diameter of the electron beam is controlled by magnitude and direction of the magnetic field. In other words, focusing of the electron beam is controlled by magnitude and direction of the magnetic field. Therefore, direct current I0 is supplied to the coil of the electromagnet, and the degree of convergence or divergence of the electron beam is adjusted by magnitude of the direct current I0.
The electron beam whose diameter has been controlled by the focusing electromagnet 16 is scanned in the x and y directions by the scanning electromagnets 17, 18. The scanning electromagnet 17 comprises an electromagnet having a pair of poles for deflecting the electron beam in the y direction, and the scanning electromagnet 18 comprises an electromagnet having a pair of poles for deflecting the electron beam in the x direction. By controlling magnitude and direction of current supplied to the coils of the scanning electromagnets 17, 18, angles of deflection in the x and y directions are controlled, and hence the electron beam is scanned and the irradiation position of the electron beam is controlled. In an example, the electron beam is scanned in the y direction (latitudinal direction) using rectangular wave in the scanning electromagnet 17, and the electron beam is scanned in the x direction (longitudinal direction) using sine wave in the scanning electromagnet 18.
However, when the electron beam is scanned in the x direction by the scanning electromagnet 18, if the angle of deflection is large in the vicinity of maximum scanning positions A, B corresponding to both scanning ends, the electron beam is deflected by the magnetic field produced by the electromagnet, so that an angle of outgoing electron beam differs according to an angle of incidence of the electron beam. Therefore, the electron beam converges at the irradiation window portions A, B corresponding to the maximum scanning positions A, B due to a lens effect created by a convex lens or the like. Specifically, as shown in the irradiation window portions A, B and C of FIG. 3, while the beam diameter is about 10 cm, for example, at the central position C, the beam diameter is about 5 cm, for example, at the maximum scanning positions A, B corresponding to both scanning ends. Thus, the irradiation area of the electron beam is remarkably converged at the maximum scanning positions A, B. The irradiation window 20 comprises a thin film made of titanium (Ti), and hence if the electron beam converges at the maximum scanning positions A, B or thereabouts, then energy density of the electron beam is increased thereat, causing damage to the irradiation window.
Further, areas where irradiation of the electron beam is not made are formed at the maximum scanning positions A, B or thereabouts, and hence toxic components in the combustion exhaust gas cannot be sufficiently removed.
It is therefore an object of the present invention to provide electron beam irradiation apparatus and method which can prevent an electron beam from being converged at a maximum scanning position and can stably obtain an irradiation area having a uniform energy density where irradiation of the electron beam is uniformly performed.
According to an aspect of the present invention, there is provided an electron beam irradiation apparatus, comprising: an electron beam source for emitting electrons; an accelerating tube for accelerating the electrons emitted from the electron beam source; a focusing electromagnet for controlling a diameter of an electron beam by applying a magnetic field to an electron beam having a high energy formed in the accelerating tube; an electromagnet for deflecting and scanning the electron beam by applying a magnetic field to the electron beam; and an irradiation window for allowing the electron beam to pass therethrough; wherein the electron beam is focused at a focus point by the focusing electromagnet so that the electron beam converges once and then diverges, and then emitted through the irradiation window to the outside.
According to the present invention, the electron beam converges once at the focus point and then diverges, and then is emitted through the irradiation window to the outside. Therefore, the beam diameter of the electron beam can be enlarged at the irradiation window. The beam diameter of the electron beam which has focused tends to be larger at the positions A, B having a large angle of deflection than that at the central position C. Thus, since the beam diameter of the electron beam is sufficiently enlarged at the maximum scanning positions (maximum angle of deflection), the convergence of the electron beam at the irradiation window portion can be prevented. Therefore, the irradiation density of the electron beam at the irradiation window portion can be uniformized to thus prevent the irradiation window portion from being damaged. Further, the electron beam is uniformly emitted through the irradiation window portion to allow combustion exhaust gas to be uniformly irradiated with the electron beam, thus removing toxic components sufficiently from the exhaust gas.
It is desirable to position the prefucus point where the electron beam converges once and then diverges at a location after the electron beam passes through the magnetic field for deflecting and scanning the electron beam in a travelling direction of the electron beam. Thus, in the case where the accelerating energy of the electron beam is so large as to be about 800 kV and the velocity of the electron beam is close to the light velocity, the electron beam becomes relativistic electron beam (REB). The present invention is applicable to such electron beam. Therefore, even if an angle of deflection is large, the beam diameter having a sufficient expansion at the irradiation window portion can be obtained.
It is desirable that the location of the focus point is adjusted by controlling current value supplied to the focusing electromagnet. Thus, by a relatively simple means for adjusting current value supplied to the focusing electromagnet, the beam diameter having a sufficient expansion at the maximum scanning position on the irradiation window portion can be obtained.
According to another aspect of the present invention, an electron beam irradiation method, comprising: controlling a diameter of an electron beam having a high energy by applying a magnetic field to the electron beam; deflecting and scanning the electron beam whose diameter has been controlled by applying a magnetic field to the electron beam with a focusing electromagnet; and emitting the electron beam through an irradiation window to the outside; wherein the electron beam is focused at a focus point by the focusing electromagnet so that the electron beam converges once and then diverges, and then emitted through the irradiation window to the outside.
With the above arrangement, even if the electron beam having a high energy is scanned at a relatively large angle of deflection, the convergence of the electron beam can be avoided, and irradiation of the electron beam can be carried out in a uniform energy density over a wide scanning area. Thus, the electron beam having a uniform energy density can be supplied to a relatively large irradiation area in an electron beam irradiation apparatus for treating flue gas, or the like. Further, a relatively large angle of deflection of the electron beam can be permitted to thus contribute to downsizing of the apparatus.